In recent years, an adaptive streaming technique has been developed in which the image quality of a video to be distributed via Web browsers is changed in accordance with the bandwidth of a network (for example, the Internet) to allow the video to be played without interruption. In general, the bandwidth of the Internet varies irregularly due to various factors such as the number of users and surrounding environments. The adaptive streaming enables continuous video playback without significant quality degradation by predicting the available bandwidth and adaptively selecting a bitstream commensurate with an available bandwidth.
The adaptive streaming technique is roughly divided into a dynamic encoding type and a pre-recorded type. In the dynamic encoding technique, a video server adaptively generates a bitstream for distribution based on the predicted bandwidth. The dynamic encoding technique is suitable for one-to-one video delivery. The video server needs to compress the video in real time based on the predicted bandwidth. On the other hand, in the pre-recoded adaptive streaming technique, the video client adaptively selects and requests a bitstream for distribution from a plurality of bitstreams provided in the video server, based on the available bandwidth derived by means of bandwidth prediction. The pre-recorded adaptive streaming technique is suitable for one-to-many video delivery. However, the video server needs to prepare a plurality of bitstreams with different bit rates beforehand.
Normally, the degree of attention paid by a user to a video on a screen is not uniform. Thus, in video delivery, in order to efficiently improve the subjective image quality of a displayed image, it is useful to preferentially enhance the image quality of a specific region, for example, an ROI (Region Of Interest), compared to the image quality of other regions. However, in order to achieve such local image quality control in the conventional pre-recorded adaptive streaming technique, the video server needs to prepare a substantial number of bitstreams. In particular, an enormous total number of bitstreams may be needed to allow the specific region to be optionally selected. The number of compression apparatuses needed, the capacity of storage, and the like increase consistently with the total number of bitstreams. This increases the costs of the video server.